This invention relates to enhancement of aperiodic transitions in a subcarrier modulated signal by reference to similar transitions in the baseband. More particularly, this invention relates to enhancement of quadrature modulated chroma of a composite color television signal by reference to luminance transitions in the signal (or chrominance transitions only in the absense of luminance transitions) and without demodulation of the quadrature modulated chroma subcarrier.
There is a recognized need to sharpen the images of television signals, particularly at transition edges where the picture changes brightness or color or, more commonly, both brightness and color. In television signal formats such as NTSC and PAL, the color information is encoded on a subcarrier which is interleaved with the baseband luminance information, so that black and white television receivers will operate acceptably upon the color signal. An inherent drawback of these formats is the limited bandwidth accorded to the color information component, called "chroma" or "chrominance". It occupies only about one third to one sixth of the bandwidth which is given to the brightness or "luminance" component in the baseband.
There are many opportunities in a color television system for the chroma information to become degraded, particularly at transitions from one hue to another. For example, this degradation has become particularly noticeable in connection with video tape and cassette recorders now widely available in the consumer marketplace.
In television pictures which depict reality, as opposed to electronically generated cartoons, titles, etc., there is a very high statistical correlation between changes in luminance and changes in chroma. This correlation has been used to great advantage to enhance or "crispen" chroma by prior art techniques.
One approach for crispening chroma is set forth in the present applicant's prior U.S. Pat. No. 4,030,121, entitled "Video Crispener". Therein, applicant discussed the prior art approaches taken by others at the time he made the invention disclosed there, and he proposed a system which, among other things, crispened chroma transitions by developing a correction signal from the luminance component. That approach, depicted in FIG. 9 of that patent and discussed in connection with that figure, called for demodulating the chroma subcarrier into its two (I and Q) components, separately enhancing the components in the demodulated baseband and then remodulating the components into the requisite color subcarrier before it could be recombined with the luminance to provide the enhanced color composite signal.
The approach depicted in U.S. Pat. No. 4,030,121 worked very well, and many units were sold which incorporated that invention. It did an excellent job of making the chrominance transition of shorter duration whenever a luminance transition was present, and it corrected chroma-to-luminance delay errors occuring prior to enhancement by realigning the chroma transition precisely with the luminance transition.
While applicant's prior approach was effective, it was very complex and expensive, and had stringent alignment requirements throughout the circuitry thereof. For example, it had about eight separate controls which interacted during the alignment process. Because of those difficult alignment constraints and the high cost of the prior approach, a need has arisen for a system which achieves the same result as the prior approach but without the complexity associated with demodulation and remodulation of the quadrature modulated chroma subcarrier.
Another drawback of the prior art approach was that in some instances the luminance transition was of an amplitude too low to yield a correction signal for a corresponding chroma transition. In such circumstances the chroma transition remained unenhanced.